Crossing over: Impossible to Possible

CROSSING OVER-
IMPOSSIBLE TO POSSIBLE

Presented by
Dr. N. Sannigrahi, Associate Professor
Department of Botany,
Nistarini College, Purulia (W.B) India

Some Common Issues
1. Father claims that the daughter’s hair texture is close to her
paternal grand-mother,
2. .Mother very often used to say that her son’s skin complexion
is like the maternal grand- father,
3. Blue eyes with curly hair very often found to exist together,
4. The different congenital diseases or almost every disease have
some connection with the hereditary in nature.
If we enlist these types of incidents or occurrences, the
number of slides are required to account of. The question
arises whether the characters transmission follow some
hereditary pattern or sometimes the parents may not
responsible for this kind of connectivity. The criss-cross
inheritance was a pleasure to every Biology students in early
school days. Let us explore the cause and consequences of the
transmission of the traits. The answer partially lies in the
understanding of impossible to possible with the courtesy of
crossing over.

The Crossing over is the most dramatic consequences in the field of
reproductive biology so far its outcome and mechanisms are
concerned. The homologous segments between non-sister
chromatids of homologous chromosomes are known as simply
crossing over. It is responsible for the recombination between
linked genes and it generally takes place during the pachytene phase
of the prophase 1 of the meiotic 1 cell division of meiosis during
the gametogenesis episode. In pachytene, each chromosome of a
bivalent ( Chromosomes pair) has two chromatids. Thus, each
bivalent contains four chromatids or strands (four –strands) stage .
Generally one chromatid from each homologue involved in
crossing over. In this process, a segment of one chromatids
becomes attached in a place of the homologous segment of the non-
sister chromatid and vice versa. Breakage and reunion experienced
by the non-sister chromatids followed by the reunion of the
acentric segments. This produces a X like figure called chiasma.

RESULT OF CROSSING OVER
The crossing over has the pleasure of the formation of two kinds of
attributes as far as the outcome is concerned- to preserve the old
combination and to promise the development of new combination.
Each event of the crossing over produces two recombinant
chromatids- cross over chromatids and the two original chromatids-
non-crossover chromatids. The cross over chromatids will have new
combinations of the linked genes, i. e recombinant. Gametes carrying
them will produce the recombinant phenotypes in test cross.
Therefore, these phenotypes are called crossover types. Similarly, the
non-cross over types will contain the parental gene combination. The
gametes carrying them will give rise to the parental phenotypes or
non cross over types. Therefore, the frequency of crossing over
between two genes can be estimated as the frequency of recombinant
progeny obtained in a test cross for these genes. The frequency is
usually expressed as per cent. The percent frequency is the frequency
of crossing over between the two genes in question.

Frequency of Crossing over= Number of recombinant progeny in
the test cross/ Total number of progeny in the test cross
Or
Frequency of crossing over (%)= Number of recombinant progeny
in the test cross/ Total number of progeny in the test cross * 100.
Now, at the very beginning of this episodes, you can recall the
social debate about the appearance of the character within their
progeny can be answered primarily by the virtue of the crossing
over phenomenon. If the recombinant one with linked genes
responsible of some characters association, then the siblings bear
some foot prints in their phonotypical expression, otherwise, the
old combinations of characters may be retained.

FACTORS AFFECTING RECOMBINATION FREQUENCY
The different factors play a very crucial role for the degree of the
chance of crossing over and this may be either due to the internal
factors and due to the external attributes upon which this
phenomenon is supposed to be happened.
1. Distance between Genes- The distance among the linked genes
of a particular chromatid play a very significant role for the
degree of the crossing over. Thus, crossing over between two
genes would increase with an increase in the distance between
them. Since the location of any two genes in a chromosome are
generally fixed or constant, the recombination frequency
between them may be also be expected to show little variation.
2. Sex- The frequency of recombination is markedly influenced
by the sex of the heterozygote for the linked genes. In general,
the heterogametic sex shows relatively lower recombination
frequencies than the homogametic sex of the same species.
Drosophila male is the good example in this regard.

3. Age of the Female- The frequency of the recombination shows a
progressive decline with the advancement of age as observed in the
female Drosophila .
4. Temperature- The frequency of recombination show a peculiar
behavior as far as the temperature is concerned. In Drosophila, the
lowest frequency of recombination is observed when the females
arte cultured at 22 ℃. The frequency of the recombination tends to
increase both the higher and the lower degree of the optimum
temperature i. e 22 ℃.
5. Nutrition- Nutrition plays a very significant role in this regard .
The frequency of recombination in Drosophila is affected by the
presence of metallic ions like Ca++, Mg +2. High Calcium diet
reduces recombination. A removal of metallic ions cause an
increase of recombination. Even starvation during the stage of the
gametogenesis may also shows notable change in the
recombination.

6. Chemicals- Certain chemicals tend to increase in recombination.
For examples, injecting the females with certain antibiotics like
mitomycin C and actinomycin D promotes recombination.
Treatment of females with alkylating agents like Ethyl methane
sulphonate (EMS) also has same kind of effect.
7. Radiation- The different short wave radiations with high energy
content like x-rays, ƛ rays and other radiation may increase the
chance of recombination. Even , drosophila males also show the
noticeable changes in the degree of recombination.
8. Plasmagenes- Some plasmagenes as present in the cytoplasm also
effect the recombination frequency. Usually, they reduce
recombination. The reason behind the plasmagenes to have an
impact upon the chance of recombination is a great matter of concern
to the biologist.
9. Genotypes- Many genes are known to effect the occurrence of the
rate of recombination.

10. Chromosomal aberrations- In Drosophila, the paracentric
inversion reduce recombination between the genes located within
the inverted segment. Therefore, such inversions are often denoted
by C crossover. Translocation- a kind of the morphological
modification of the chromosomes in the vicinity point of the
crossing over may also reduce the chance of recombination.
11. Distance from the centromere- Centromere, the point of the
attachment of the arms of the chromosomes tends to suppress the
chance of recombination. Therefore, the genes located in the
vicinity of the centromere shows relatively lower recombination
rate than those are far off the vicinity of the centromere.
In addition to these aforesaid factors, there are other number of
factors play a very important role in the magical events of crossing
over to make impossible to possible in the acceleration of the
passage of evolution.

TYPES OF CROSSING OVER
The crossing over may be classified under different headings-
1. On the basis of the nature- Somatic Crossing over & Germinal
crossing over
2. On the basis of the degree of the formation of chiasm-
Single cross over, Double Cross over &Multiple cross over
Single Crossing Over:
It refers to formation of a single chiasma between non-sister
chromatids of homologous chromosomes. Such cross over involves
only two chromatids out of four.
ii. Double Crossing Over:
It refers to formation of two chiasmata between non-sister
chromatids of homologous chromosomes. Double crossovers may
involve either two strands or three or all the four strands. The ratio
of recombinants and parental types under these three situations are
observed as 2:2:3:1 and 4 : 0, respectively.

iii. Multiple Crossing Over:
Presence of more than two crossovers between non-sister
chromatids of homologous chromosomes is referred to as multiple
crossing over. Frequency of such type of crossing over is
extremely low.
SOMATIC CROSSING OVER
This is very unusual one and this type of crossing over involved
during the somatic cell division of mitosis cell division.
1. Somatic Crossing-Over In genetics, crossing-over during
mitosis of somatic cells such that parent cells heterozygous for
a given allele, instead of giving rise to 2 identical heterozygous
daughter cells, give rise to daughter cells one of which is
homozygous for one of these alleles.
2. Germinal Crossing Over- This is very common one which is
really advocated as the true crossing over. In this condition, the
non-sister chromatids of the two homologous chromosomes
are invo0lved for the exchange of the materials in search of
the recombination for the nest generation after the fertilization.

MOLECULAR MECHANISM OF CROSSING OVER
The mechanism of crossing over apparently seems to be very easy
process where the two non-sister chromatids undergo breakage
followed by the reunion to exchange the chromatid segments but
the molecular mechanisms behind this process is quite complex
and it deserves the understanding a series of complicated process
for the same. Different molecular biologists have put forward a
number of theories in this regard but the most advocated theory is
the copy choice theory & breakage and reunion theory expedited
by the different molecular scissors like enzymes for doing this
impossible to possible one. The detail steps of breakage and
reunion theory is stated as below:
This theory advocates that crossing over takes place by breakage
and reunion of the two non-sister chromatids. The entire
biological process needs the involvement of the different enzymes
in this regard.

Molecular models of Recombination is of broadly two
categories-
Hybrid DNA models involving single stranded break
Hybrid DNA models having two strands break.
HYBRID DNA MODELS INVOLVING SINGLE STRAND
BREAK
This molecular mechanism assume breaks in only one of the two
strands on each of the two DNA molecules belonging two non-
sister chromatids and this has been suggested by H.L.K
Whitehouse (1963) of Cambridge and other by Robin Holiday of
London ( 1064).
HOLIDAY MODEL
This is most common among the homologous recombination
models and this has received wide support both for the
prokaryotes and eukaryotes. The entire process takes place
sequentially as par stated below:

1. A paired bivalent in meiosis consists of four chromatids at a
particular location, two of them take part in recombination
although for double or multiple crossing over, three or all the
four chromatids may be involved.
2. The process starts with breakage at corresponding points of the
homologous strands of two paired duplex. Other two chromatids
are being not involved in this condition.
3. The breakage allows movement of free ends created by the
nicks. Each strand at the broken end leaves its partner, cross
over, by displacing the broken strand of the other non sister
molecule and its pairs with its complement into the other DNA
duplex. Thus, it creates a connection between the two DNA
duplexes.
4. 4. initially, this connection is sustained by Hydrogen bonding
but at some points, branch migration takes place and nicks at
the sites of exchange are sealed with the help of ligase enzyme.
The connected pair of duplex is called a joint molecule and the
point at which an individual strand of DNA crosses from one
duplex to the other is called recombinant joint.

5. At the site of recombination, each duplex has a region consisting of
one strand from each of the two parental DNA molecules .This region
is called Hybrid DNA or heteroduplex DNA.
6. One of the two DNA duplexes in the joint molecule undergoes
rotation giving rise to a planar molecule. In each planar molecules,
there are four strands and two of them are nicked.
7. The consequence of nicking will depend upon, which pair strand is
being nicked?
8. If nicks are made in the pair of strands that are not ordinarily nicked,
leads to a release of recombinant DNA molecules. The DNA duplex
segment of one parent is covalently nicked to the DNA duplex
segment of the other parent via a stretch of heteroduplex DNA. This
means that conventional recombination has taken place between the
markers located on either sides of the heteroduplex region.
9. If the two strands involved in the original nicking are nicked again,
then only the original parental duplex with heteroduplex segments
with no recombinant DNA molecules are released marker genes are
released.

HYBRID DNA MODEL
General recombination is initiated by special endonuclease that
simultaneously cut both strands of the double helix, creating a
complete break in the DNA molecule. This intact DNA is used as a
template to repair the break of DNA synthesis. The model of the
recombination develop from the studies in the S, cerevisiae and the
entire process takes place in the following sequential orders .
1. A double strand breaks in one of the paired homologous
double stranded DNA molecule by endonuclease enzyme.
2. The break is enlarged to gaps by the action of 5’-3’ exonuclease
resulting in single stranded 3’ end on both strands.
3. One of the two broken single strands invades the other duplex
so that the duplex molecules become connected by stretch of
heteroduplex DNA.
4. The heteroduplex DNA then generates a D-loop which is
extended by repair synthesis to cover the entire length of the
gap. The elongation at the 3’ end is conducted by DNA
polymerase enzyme using D-complementary strand as

5. The single stranded D-loop becomes double stranded once again by
repair synthesis.
6. Branch migration converts this structure into a molecule with two
recombinant joints , which are resolved by cleavage of the strands.
This lead two possibilities-
a. non-cross over with heteroduplex DNA,
b. B. crossing over with heteroduplexs DNA,
c. Therefore according to this model, there is an initial loss of
nucleotides which is recovered by the synthesis of DNA.
The model can account for non-Mendelian segregation of certain
markers that has been observed during meiotic recombination. In a
cross of multiple marked Yeast strains, most allelic markers
segregate according to the Mendelian ratio 2:1 but those located
near the cross over point exhibit 3:1 segregation. Such non-
reciprocal event is called gene conversion become one allele is
apparently converted to another.

SIGNIFICANCE OF GENETIC RECOMBINATION
The Crossing over is the most important biological drama acted by a
number of molecular actor to execute this event to make something
apparently impossible to make possible. Some of the significances are
as stated below:
1. Crossing over links together all four homologous chromatids and
this linkage is essential for the proper segregation of chromosomes
during meiosis. Recombination can occur with equal probablity at
almost any point along the length of the two homologous
chromosomes. The frequency of crossing over in a region
separating the two points on a chromosome is therefore
proportional to the distance between the points . Bu using the fact,
geneticists had tried to map the relative position and distance
between the genes on the chromosomes.
2. .Recombination by crossing over is a source of genetic diversity.
Recombination during meiosis in the germ cells produce an
opportunity for the exchange of the genes and thus causes the
genetic variations among the species.

3.Recominations by the courtesy of the crossing over provides an
avenue for accurate DNA repair. In certain type of DNA lesions, such
as double strand breaks, double strand cross links or lesions left
behind in a single strands during replication, the complementary
strand is itself damaged These kinds of damage commonly result
from ionizing radiations and oxidative reactions and their repair is
critical to the production of viable gametes in eukariotes.Repair is
mediated by homologous genetic recombination is simply known as
recombination repair.
The recombination via crossing over ensures the progressive
evolution of the different organisms by creating a sustainable
opportunity of variation and this is highly solicited in this regard for
the sake of the evolution and crossing over acts as acceleration in the
long drive of the highway of the evolution.
Thus, in a word, crossing over can be regarded as the pleasure of
happiness among the organisms for the blessed life.

CYTOLOGICAL BASIS OF CROSSING OVER
Experimental evidences can support the crossing over mechanisms
as executed by Stern on Drosophila and Creighton & McClintock on
maize. The linked genes are located on the same chromosome and
they would show recombination only when there is exchange of the
concerned segments between the homologous chromosomes. The
experiment on Drosophila as done by Stern as stated below:
1. Drosophila female has one X chromosome shorter than normal
and it had recessive gene car ( carnation eye color) and dominant
gene B (Bar eye shape). The other X chromosome was a normal
in length and it had the dominant gene car+ ( wild type of allele
of car; produces dull red eye color) and the recessive gene B+
(wild type of allele of B; produces normal ovate eye shape).
2. Stern test crossed this female to a car B+ male;
3. As expected, the following four types of flies were recovered in
the test cross progeny; red, normal (car+, B+) ; red, Bar (car+,
B); red, Bar ( Car+, B); carnation, normal ( car B+); carnation,
bar (car+, B)

4. Two of these four phenotypes viz. red, normal and carnation, bar
are non crossover or non-recombinant types. Therefore, the
carnation, bar individuals are expected to carry one short
chromosome while the red, normal flies would have one X
chromosome with an attached Y segment. In contrast, the remaining
two phenotypes viz. red, bar and carnation normal, are crossover or
recombinant types. If crossing over involves an exchange of
homologous chromatin segments between homologous
chromosomes, then one X chromosome of these individuals would
be the product of exchange. Therefore, carnation, normal flies (car
B+) are expected to have a normal or long X chromosome without
the attached Y segment. In contrast, red, bar individuals (Car+ B)
will have one short chromosome with the attached segment. Stern
concluded that during meiosis, there is a exchange of precisely
homologous chromatin segments between homologous chromosome
(Crossing over) and that crossing over is responsible for
recombination between linked genes.

CONCLUSION
Thus, we see that the crossing over through a series of
complicated biochemical changes lead the inclusion of the
different unrelated characters in a single canopy of the
chromosome and this is reflected in terms of the phenotypic
expression in the due course when the organisms give the birth
of the offspring as a part of sexual reproduction. Thus
apparently impossible characters and its expression becomes
possible by this unique and most interesting biological drama.
The recombinants bring the new combination while the paternal
or maternal non-recombinant chromosomes may repeat the same
characters which was supposed to be present the predecessor
generations. The linkage map or chromosome map can help us
to draw the line of the location of the genes along the length of
the chromosome with the help of the frequency of
recombination the coefficient of coincidence indicates the
degree of agreement between the observed and the expected
frequencies of double crossovers.

The occurrence of crossing over in one region of a
chromosome may interferes with its occurrence in the
neighboring segments reflected by interference. It may be
expected that the intensity of interference would progressively
decrease as the point of the second crossing over becomes
farther from that of the first one., Therefore, the coefficient of
coincidence would be lower when the concerned genes are
located close to each other than when they are located far
apart. In a word, the magic of the reality is reflected in the life.

References:
1. Google for images,
2. Principles of Genetics- Basu & Hossain,
3. A textbook of Botany (Vol III) Ghosh,
Bhattacharya, Hait
4. Fundamentals of Genetics- B.D. Singh,
5.A Textbook of Genetics- Ajoy Paul
DISCLAIMER:
This presentation has been made to enrich open
source of information without any financial interest.
The presenter acknowledges Google for images and
other open sources of knowledge to develop this
PPT.

Crossing over: Impossible to Possible

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